In general, high-voltage integrated circuits (ICs) in which at least one high-voltage transistor is arranged on the same chip together with low-voltage circuits are widely used in a variety of electrical applications. Breakdown voltage and on-resistance (Ron) are two important characteristics of a MOSFET when used in a power switch circuit. Improving the operation of a power switch circuit incorporating MOSFETs suggests using a MOSFET with a breakdown voltage as high as possible and an on-resistance as low as possible. However, low on-resistance and high breakdown voltage parameters are contradictory to each other in current process technologies.
A lateral power MOSFET is basically a metal oxide semiconductor field effect transistor fabricated with coplanar drain and source regions. A problem with this type of lateral power MOSFET is that it cannot maintain a low on-resistance when a high voltage is passed through the lateral power MOSFET. The on-resistance is the power of the current that is transformed into heat as the current travels through the device. The larger the on-resistance of the device, the less efficient the device. Accordingly, a field ring (a p-ring structure) is inserted in an N well region beneath the field oxide region to reduce the surface electrical field and improves the depletion capability of the drift region. As a result, the doping concentration of the drift region can be increased and the on-resistance of the device can be decreased. However, the breakdown voltage is still not good enough to endure the power spikes. The breakdown voltage is the voltage at which a normally high-resistance element (such as a MOS capacitor or reverse biased p-n junction) allows current to flow. When voltage larger than the breakdown voltage is passed through devices, catastrophic and irreversible damage is done to the devices, rendering the devices commercially useless and requiring the devices to be replaced. Accordingly, increasing the breakdown voltage is highly desirable.
There is a trade-off relationship between breakdown voltage and on-resistance. For switching power applications, lower on-resistance means higher efficiency, and higher breakdown means higher tolerance for power spikes. What is needed in the art, therefore, is a novel MOSFET with a reduced on-resistance and a higher breakdown voltage when the device is placed under a high voltage.